How Composition Determines the Properties of Isodimorphic Poly(butylene succinate-ran-butylene azelate) Random Biobased Copolymers: From Single to Double Crystalline Random Copolymers

Abstract

The structure, morphology and thermal properties of biobased and biodegradable poly(butylene succinate-ran-butylene azelate) random copolyesters with a wide composition range were studied. These PBS-ran-PBAz copolymers can crystallize in the entire composition range despite being random, displaying a eutectic point when their melting point is plotted as a function of composition. Wide Angle Xray Scattering (WAXS) studies confirmed isodimorphic behavior where PBS-rich copolymers crystallize with PBS type unit cells with some PBAz repeating units inclusions and vice versa for PBAz-rich copolymers. Away from eutectic compositions the copolymers exhibit only one crystalline phase (PBS-rich or PBAzrich crystalline phases) while at the eutectic point both phases can crystallize. The only double crystalline copolymer amongst those prepared had a composition close to the eutectic point of 45 mol% PBS (and 55 mol% PBAz). The crystallization of the two phases occurred in the same temperature range upon cooling from the melt at 10ºC/min in a DSC (i.e., coincident crystallization). Self-nucleation (SN) studies of the PBS-rich phase were performed. After SN, the separate crystallization of each phase was clearly observed during cooling from the self-nucleation temperature (i.e., PBS and PBAz-rich phases). Small Angle X-ray Scattering (SAXS) experiments were performed for the first time for this type of isodimorphic copolymers. The results show that the lamellar long period is a strong function of composition. While limited inclusions of PBAz units within the crystal lattice only cause a slight expansion of the PBS component unit cell, the increase of comonomer content produces an unexpected synergistic increase in long periods and lamellar thickness values. In the case of the only double crystalline copolymer examined, the PBS-rich phase forms space filling spherulites (observed by Polarized Light Optical Microscopy, PLOM) at higher temperatures that template the superstructural morphology of the copolymer. These PBS-rich phase spherulites contain radial lamellar stacks whose long period was determined by SAXS. Upon further cooling, the PBAz-rich phase crystallizes in the intra-spherulitic amorphous regions with newly form lamellae that have their own distinct long period according to SAXS results. AFM observations of the PBS-rich crystalline lamellae confirmed the lamellar thickness and long spacings determined by SAXS. A schematic morphological model of the mixed spherulites produced by this double crystalline diblock copolymer is proposed based on the experimental evidences collected by SAXS, PLOM and AFM.

Nucleation and crystallization of random aliphatic-butylene terephtalate copolyester

“…In the PBASGT case this Domain is located entirely at the baseline beyond the melting endotherm (see Fig. 4) as it was found also for poly(e-caprolatone) [27] and poly(butylene succinateran-butylene azelate) [28]. Strong and distinct self-nucleation suggests that nucleating agents for crystallization of PBASGT can be found.…”

Self-Nucleation of Crystalline Phases Within Homopolymers, Polymer Blends, Copolymers, and Nanocomposites

“…In domain III, the temperature is low enough to partially melt the polymer, while the unmolten crystals anneal into thicker crystals. Figure 4 illustrates the SN behavior of poly(butylene succinate) (PBS) [8]. Two polarized optical micrographs show the typical morphology after cooling the sample from the melt in domain I (after the melt memory is erased) or from domain II (after SN).…”

“…3 can be used to illustrate a strong crystalline memory effect. It is clear that crystalline memory is the only explanation for the behavior of the material in the high T s region within domain II, because full melting had been attained, as judged by the return of the DSC trace to the base line after melting at temperatures above 120 C. Wide-angle X-ray scattering (WAXS) experiments also indicate complete melting at temperatures above 120 C [8].…”

“…This process erases the memory of previous processing and thermal treatments. The memory effect could originate from some embryos (molecular clusters) that survive the thermal treatment and then Reprinted with permission from Arandia et al [8]. Copyright (2015) American Chemical Society become active nuclei once the temperature is lowered [26,44].…”

“…Coincident and fractionated crystallization phenomena are frequently observed in blends and copolymers, and SN can be employed to investigate the origin of the crystallization process [3,8,[87][88][89][90][91][92][93][94][95][96][97][98][99][100]. The work performed by Morales et al [93] has been chosen as an example of the application of SN to a polymer blend.…”